Background: Heat stress poses a serious risk to aquaculture by impairing fish health and increasing mortality. Functional feeds are increasingly recognized as practical nutritional tools to counteract the detrimental effects of thermal stress.

Objectives: This study evaluated the effects of dietary supplementation with intact and fermented green cumin seed on the immunological and antioxidant responses of common carp (Cyprinus carpio) subjected to heat stress.

Methods: Green cumin seeds were obtained locally and fermented using baker’s yeast. Experimental diets were prepared by supplementing a basal diet with 5 or 10 g/kg intact green cumin seed or fermented green cumin seed. Juvenile common carp (10.3 ± 0.13 g) were fed the experimental diets for 8 weeks and were then subjected to acute heat stress at 37℃ for 4 days. Skin mucus samples were collected at the end of the feeding trial, immediately before the heat challenge, and blood samples were collected both before the heat challenge and 24 h after the onset of heat exposure for determination of immune and antioxidant parameters.

Results: Dietary green cumin supplementation, particularly in fermented form, significantly enhanced skin mucus immune responses in common carp. Fish fed fermented green cumin showed higher bactericidal, lysozyme, protease, natural hemolytic, peroxidase, alkaline phosphatase, and total immunoglobulin activities than the control group, with the strongest responses generally observed in the fermented treatments. Following heat stress, the relative survival percentage increased in all supplemented groups, with the highest value observed in fish fed 5 g/kg fermented green cumin, followed by those fed 10 g/kg fermented green cumin. Regardless of heat stress, functional diets improved plasma antioxidant status by increasing superoxide dismutase, glutathione peroxidase, reduced glutathione, ascorbate, and ferric reducing antioxidant power, while decreasing malondialdehyde concentration. Heat stress itself increased plasma superoxide dismutase, catalase, glutathione peroxidase, and reduced glutathione, but decreased ascorbate and ferric reducing antioxidant power.

Conclusion: Fermented green cumin exerted stronger immunostimulatory, antioxidant, and stress-protective effects than intact green cumin. Overall, dietary fermented green cumin enhanced mucosal immunity, improved antioxidant defense, reduced lipid peroxidation, and increased resistance to heat stress in common carp.